[klibc] sparc64: transmit arch-specific options to kinit via /arch.cmd

Sparc64 has support for a bunch of architecture-specific options
taken from the PROM.  Convert them to kernel command line form and
write the file /arch.cmd in the rootfs, which can be recovered by
kinit and processed.  The kinit support has already been committed.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>